This invention relates generally to power conversion systems and more particularly to a system for controlling an AC motor by controlling the motor current via a three phase converter-inverter system.
While DC motors historically have had widespread use where operation over a wide speed range is desired, more recently AC motors have been finding greater application in variable speed drive applications. There are, however, certain problems associated with the use of AC motors particularly where the motor is supplied with power from a system comprising an AC to DC converter feeding a variable frequency DC to AC inverter such as a phase controlled thyristor inverter. As disclosed, for example, in the above referenced U.S. Pat. Nos. 4,156,896 and 4,164,015, if one is operating a phase controlled thyristor AC to DC converter in a negative region (fourth quadrant) and a positive command voltage is suddenly applied, the output can move into the positive region (first quadrant) relatively quickly. However, if one is operating in the positive region and a reverse command voltage is applied, transition to the negative region cannot take place as rapidly due to the inherent transport lag or delay time encountered in commutation. Thus, in systems where the DC voltage is required to change rapidly in positive and negative directions, response to forward and reverse command signals will not be the same due to the inherent unequal bidirectional operating characteristic encountered with phase controlled thyristor bridge converters.